Automatic tuning apparatus and method for substance detection using nuclear quadrupole resonance and nuclear magnetic resonance

ABSTRACT

Apparatus for tuning a system for detecting a target substance through the use of nuclear quadrupole resonance (NQR) or nuclear magnetic resonance (NMR). The apparatus tunes an RF coil (34) which is employed to excite the substance under test. The tuning apparatus (36) includes a series of fixed value capacitors (93) switched in the tuning circuit by controllable switches (94). A programmable controller (21) is employed to control the switching and thereby the tuning of the coil.

This is a continuation of application Ser. No. 08/400,673, filed Mar. 8,1995, now U.S. Pat. No. 5,594,338, issued Jan. 14, 1997.

TECHNICAL FIELD

This invention relates generally to a bulk substance detection systemsfor detecting concealed explosives and narcotics employing eithernuclear quadrupole resonance or nuclear magnetic resonance, and moreparticularly to a practical system and method for tuning such detectionsystems.

BACKGROUND ART

Certain atomic nuclei, typically having a spin quantum number of 1/2,exhibit magnetic signatures when they are within an externally appliedmagnetic field. This magnetic resonance effect is most commonly observedin ¹ H, and is known as nuclear magnetic resonance (NMR). Atomic nucleiwith a spin quantum number of >1/2 can also show another magneticsignature associated with the interaction of the nuclei with the localelectric field. This phenomenon is known as nuclear quadrupole resonance(NQR).

For both of these phenomena, the energy level transitions are observedprimarily in the radio frequency range. Detection of these transitionsthus requires a radio frequency source to excite the transition, and aradio frequency receiving mechanism to detect the signal. Normally, thesignals appear at a pre-defined frequency. An RF coil tuned to, or closeto, that predefined frequency can excite or detect those signals. Thesignals are of very low intensity and can only be observed for a shorttime. approximately 10 μs to 2 ms. As a consequence, there is a need foran NQR or NMR receiver that can be tuned to (usually) high Q, has verylow noise, and is capable of fast recovery after a high voltage RFpulse. In most conventional magnetic resonance (NMR and NQR)experiments, small and fairly homogeneous samples are investigated.

Over the past few years there has been considerable interest in thelarger-scale "real world" applications of both NQR and NMR. Theseapplications do not benefit from the luxury of small-scale laboratoryinvestigations. They usually require investigation of large volumesfilled with materials of vastly differing physical and chemicalcomposition. Investigation of the contents of mail or baggage for thepresence of explosives or narcotics is an example.

With respect to explosives, plastic explosives such as C-4 and Semtex,containing RDX and PETN, have an almost infinite variety of possibleshapes and uses for terrorist bombing tactics. Plastic explosives arehighly stable, have clay-like malleability and are deadly in relativelysmall quantities. A small piece of plastic explosive, a detonator, and atrip wire inside a large mailing envelope can cause a deadly explosion.Unfortunately, without close--and potentially dangerous--visualinspection, plastic explosives can be made virtually untraceable. Inparticular, detection of sheet explosives, typically having a thicknessas small as one-quarter inch, has not been effectively accomplished byprior technologies.

The wide-scale attempts to fight the illegal drug trade indicates thatnarcotics detection is also extremely important. The need for a simpleprocedure for detecting drugs inside sealed containers, mail parcels,and other small packages, quickly and accurately, is immeasurable.Conventional detection methods are time-consuming, costly, and have onlymarginal reliability at best.

Detection by means of NQR or NMR is possible for both explosives andnarcotics, partially because they have as a constituent element ¹⁴ N incrystalline form. Particularly with respect to narcotics, this is trueof cocaine base, cocaine hydrochloride and heroine based narcotics. Thehydrochloride forms of narcotics, such as cocaine hydrochloride, alsocontain quadrupolar nuclei ³⁵ Cl and ³⁷ Cl.

A significant factor in contraband detection by means of NQR inparticular is that quadrupolar nuclei that are commonly present, andpotentially readily observable, in narcotics and explosives includenitrogen (¹⁴ N) and chlorine (³⁵ Cl and ³⁷ Cl), among possible othernuclei. Thus, in commercial applications it is necessary to be able todetect quadrupolar nuclei contained within articles of mail, mail bagsor airline baggage, including carry-on and checked luggage. While theresonant frequencies of the nitrogen in these substances differs foreach chemical structure, these resonant frequencies are well defined andconsistent. By applying an RF signal to a container having any of thesesuspected substances inside, and then detecting any quadrupolarresonance thus engendered by the application of RF pulses, the identityof the contraband substance can be easily determined.

NQR and NMR signals originate from the energy transitions associatedwith certain nuclei. These energy transitions are usually in the radiofrequency range. Thus, detection of both NQR and NMR signals normallyrequires the use of radio frequency transmitting and receivingapparatus. To minimize noise and radio frequency power requirements andimprove receiver sensitivity, conventional NQR and NMR systems use anarrow band, high Q, sample coil for both transmitting and receiving.There are, however, a number of factors that can significantly degradethe effectiveness of detecting NQR and NMR signals using this kind ofnarrow band, high Q, detection apparatus. Some of them are:

(1) the presence of large conductive materials inside the sample coil;

(2) the presence of materials with high dielectric constant inside thesample coil;

(3) temperature, which can affect the value of the capacitance used fortuning and matching the RF coil; and

(4) mechanical movement of the coil with respect to its surroundings.All of these factors can cause serious de-tuning of the detectionapparatus, which in turn causes a reduction in the detection sensitivityof NQR and NMR signals from the materials inside the sample coil.

Previously, for most applications of NQR and NMR, these conditions havenot presented a serious drawback. The apparatus could usually be set upunder near-optimum conditions, and the materials being investigated wereusually well characterized. However, over the past few years several newapplications have arisen which require NQR and NMR apparatus and methodsfor the detection of certain materials under adverse conditions (forinstance, applications in which large volumes of largely uncharacterizedmaterials are under investigation).

A system intended for use in nuclear resonance experiments involvingfrequency sweeps is described in Butler et al.. High-Power RadioFrequency Irradiation System with Automatic Tuning, Rev. of SciInstruments, Vol 53, No. 7, pp 984-988 (1982). This reference teachesadjusting the frequency of the system to a known frequency by means of alook up table. A multi-turn tuning coil wound on a glass mandrel isemployed.

U.S. Pat. No. 5,208,537 is directed at a method for matching antennas inan NMR imaging apparatus. Like Butler above, this reference builds alook up table specific to each patient for which a tomogram is beingproduced.

DISCLOSURE OF INVENTION

Broadly speaking, this invention provides a practical method andapparatus for improved methods for bulk detection of substances usingnuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)under less than optimum conditions. The invention is a method of, andapparatus for, automatic fine tuning of an NQR or NMR detectioncoil/head under such adverse conditions. The system consists of a seriesof fixed value capacitors switched by vacuum relays. The amount ofcapacitance switched into the tuning circuit is determined by measuringthe amount of power being transferred from the RF amplifier to the RFdetector coil (or more precisely the amount of "forward" to "reflected"power). The means to measure this power transfer efficiency consist of avariety of common RF techniques. For one application, a directional wattmeter is used to measure the amount of "forward" to "reflected" power.Based on the power transfer efficiency, capacitors are switched in orout of the circuit to maximize power transfer efficiency from the RFamplifier to the RF coil. The system is thus re-tuned to provide themost efficient and most sensitive RF coil.

The invention in its essence is an automatic tuning apparatus toestablish maximum power transfer efficiency at at least one selectedpredetermined frequency in nuclear quadrupole and nuclear magneticresonant (NQR/NMR) detection systems when a specimen is inserted intothe detection element of an NQR/NMR detection system in order to enablesuch detection systems to detect nuclear quadrupole or nuclear magneticresonant frequencies of predetermined substances if present in thespecimen, the predetermined substances having predeterminedcharacteristic nuclear resonant frequencies, wherein nuclear resonantfrequency is the frequency of nuclear presession due either toquadrupole interaction with molecular electric field gradients (NQR) orto an applied static magnetic field (NMR), said apparatus comprising: asequence controller having means for providing timing and programmingpulses to said apparatus; a variable frequency RF source to providepulsed RF exitation at a predetermined frequency generally correspondingto the nuclear resonant frequency of a predetermined substance; a singleturn distributed RF coil sheet shaped and configured to define a cavityof pre-determined volume therewithin and to receive the specimen withinthe cavity defined by said coil, the RF signals from said RF sourcebeing transmitted within said cavity and being uniformly applied to thespecimen within said RF coil cavity and generating a uniform fieldwithin said cavity, said RF coil also functioning as the pickup coil forthe NQR/NMR signals from the specimen and providing an output signal;means for measuring the power transfer efficiency of said RF coil at thepredetermined nuclear resonant frequency; an array of fixed valuecapacitors connected in said apparatus and selectively switchable intocircuit with said RF coil; and a plurality of individually controllableswitch means for selectively connecting said capacitors into said RFcoil circuit; said sequence controller controlling the switching of saidcapacitors into and out of circuit with said RF coil pursuant to powertransfer efficiency measurements from said measuring means to establishand maintain maximum power transfer efficiency of said RF coil at thepredetermined nuclear resonant frequency.

The invention also includes a method for automatically tuning apparatusto establish maximum power transfer efficiency at a predeterminedfrequency in a coil when a specimen is inserted into a cavity formed bysaid coil to enable said coil to detect nuclear quadrupole or nuclearmagnetic resonant (NQR/NMR) frequencies of predetermined substances ifpresent in the specimen, the predetermined substances havingpredetermined characteristic nuclear resonant frequencies, whereinnuclear resonant frequency is the frequency of nuclear precession dueeither to quadrupolar interaction with molecular electric fieldgradients (NQR) or to an applied static magnetic field (NMR), saidmethod comprising the steps of: shaping and configuring a single turn RFcoil sheet to define the cavity having a predetermined volumetherewithin, the cavity being adapted to receive specimens to be testedfor the presence of the predetermined substances; inserting the specimenwithin the cavity formed by the RF coil; providing RF pulses at aboutone said predetermined characteristic nuclear resonant frequency to theRF coil to establish a uniform field within the cavity in said coilwhich encompasses the specimen to thereby uniformly apply RF signals tothe specimen within the RF coil cavity; measuring the power transferefficiency of the coil at the predetermined nuclear resonant frequency;and selectively switching a plurality of fixed value capacitors intocircuit with the RF coil to establish maximum power transfer efficiencyof the coil with the specimen therewithin at the predetermined nuclearresonant frequency.

BRIEF DESCRIPTION OF DRAWING

The objects, advantages and features of this invention will be morereadily appreciated from the following detailed description, when readin conjunction with the accompanying drawing, in which:

FIG. 1 is a block diagram of an NQR detection system which includes theautomatic tuning apparatus of the invention.

FIG. 2 shows the auto-tune subsystem in greater schematic detail; and

FIG. 3 is a flow diagram of the operation of the auto-tune subsystem ofFIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The method and apparatus determine the state of the tune of the RF coilin the detector head by measuring the amount of power being directlytransferred to the RF coil (the "forward" power), and the amount ofpower being lost due to losses in the circuit and mis-tuning (the"reflected" power). Once the state of tune of the RF coil has beendetermined by the values of the forward and reflected power, the coil isre-tuned by switching in capacitance according to the algorithmdescribed below.

Before describing the tuning apparatus of the invention in detail, FIG.1 will be described. This is an NQR detection system which includes thetuning apparatus of the invention as an integral and significant part.Here is shown the detection system where block 21 is a sequencecontroller subsystem. This subsystem provides precise timing and othercontrol functions for all other elements and subsystems of theinvention. It generally would comprise a microprocessor-based devicewhich provides means to download and initialize the sequence controlinformation to all other subsystems, and would include appropriate datastorage or memory means. It also stores information on the results ofindividual scans for future reference. As one specific embodiment, themicroprocessor based control and storage device may be a personalcomputer (PC) with a hard disk.

The sequence controller subsystem also includes a pulse programmer whichis a high-precision, high-resolution device that runs off the standardcomputer bus. The pulse programmer provides the precise sequence controlrequired for correct operation of all other major components in the NQRscanner shown. In combination with the personal computer, it alsoprovides the precisely defined pulses and triggers to activate thesubsystems to which it is connected and which will be discussed indetail below.

Radio frequency (RF) subsystem 22 has several functional elementsincluding RF signal source 23, RF power amplifier 24, receiver RFpreamplifiers 25, receiver RF amplifier 26 and detectors 27 and 28. Thedetectors are here shown as phase-sensitive detectors. A 90° degreephase shift generator 31 is also part of the RF subsystem. This is oneembodiment of the invention and is used when detectors 27 and 28 arephase shift detectors. Conventional amplifier protection devices 29 arealso part of the RF subsystem.

RF signal source 23 provides either continuous or pulsed RF excitationat a frequency corresponding to the resonant frequency of the samplematerial. For example, RDX-based plastic explosives have a resonantfrequency of approximately 3,410 MHz while PETN-based plastic explosiveshave a resonant frequency of approximately 890 KHz. The excitationsource is fed into amplifier 24 of sufficient power rating to generateabout 1 gauss of RF magnetic field within the coil. The excitationfrequency need not be exactly the same as the target substance NQRfrequency but should be within about 500-1000 Hz. The RF excitation forNQR detection could be a single pulse of 10 μs-500 μs duration,depending on the substance being tested for. Such a single pulse couldcause an NQR return, but the nuclei may not have reached a steady stateof precess so the NQR return might not be sufficiently strong to bedetectable or useful. For a letter bomb scanner, approximately threeseconds of RF pulses at a repetition rate of 667 pulses per second,meaning a train of 2000 pulses having a pulse width of 200 μs each,would preferably be applied. The frequency of these pulses can rangebetween 300 Hz and 2 KHz. This would result in a series of NQR signalswhich are added and averaged in digital signal processor 44. This is anapplication of the conventional technique where target signals are addedlinearly while noise adds randomly, thereby building a clearly definablepulse by improving the signal-to-noise ratio (SNR). Any method toimprove SNR might advantageously be used.

The power requirements of the NQR scanner are generally proportional tothe detection coil volume. An explosives scanner for mail packages witha 25 liter detector coil volume might have an RF power amplifier ratedat about 25 Watts, peak value, for example. The amplifier produces auniform RF field of about 1 gauss over the entire 25-liter volume. Inother application, such as in narcotics detection, the RF field may begreater than this value. For airline baggage, an explosives detectionhead of about 300 liters (10 ft³) requires a 1 to 2 KW RF poweramplifier. These parameters are provided for reference purposes and arenot meant to define or limit the actual characteristics of a practicalNQR system.

The RF excitation pulses are fed from amplifier 24 into detection head33, the operation of which will be discussed below. After the sample inthe detection head has been excited by the RF pulse, a short RF coil"ring-down" or dead time occurs, during which the receiver is "deaf,"before sensing occurs. This ring-down time could, for example, be 500μs. Then RF coil 34 detects the NQR signals and the response isamplified by low-noise, high-grain preamplifier 25 having a gain of 20to 30 dB, and a noise figure of 1 to 2 dB. Examples of suchpreamplifiers are Anzac Model AM-110 and Mini-Circuits Model ZFL-500LNS.

In the package or letter scanner size configuration of the invention,after the received signal has been sufficiently amplified by RFamplifiers 25 which, together with amplifier protection components 29,include appropriate conventional filter functions, it is fed into twophase sensitive detectors 27 and 28, having reference signals shifted90° from each other by means of phase shift element 31. Note thatreference RF signal from RF source 23 is applied to phase sensitivedetector 27 while the reference signal to phase sensitive detector 28passes through phase shift element 31. The two mutually phase-shiftedanalog signals are then fed into signal-capture and data processingsubsystem 41, which will be discussed below.

Detection head subsystem 33 is comprised of four main components. Theseare RF coil 34, and RF probe circuit which is RF tuning and matchingnetwork 35, RF shield 37, and auto-tune subsystem 36 of this invention.The detection head serves two primary purposes. One is to produce ahomogeneous RF field in the RF coil. The other is to receive the raw NQRsignal, if present, from the item under investigation.

RF coil 34, which may also referred to as an antenna, is made of ahighly conductive material, such as copper. The conductor should have athickness in the order of at least five times the skin depth of thematerial of the conductor at the operational frequency. This ensures aminimal amount of resistance to the flow of current when the coil isenergized with RF. A 25 liter detection volume (for a mail scanningdevice) has a single turn, high-Q, 0.254 mm (0.010 inch) thick coppercoil made of a single sheet. The skin depth of copper at 3.4 MHz isabout 0.0254 mm (0.001) inch and the skin depth of copper at 900 KHz isabout 0.0508 mm (0.002 inch).

Direct coil tuning results in an increased overall efficiency for themail scanning embodiment of the invention. This tuning is accomplishedby subsystem 36 of this invention. The single-turn, high-Q coil, when nosample is present, that is, the coil is empty, requires approximately30,000 pF of capacitance for tuning at about 3.4 MHz in order to detectthe ¹⁴ N resonant frequency of RDX explosives. Using a series ofswitches to add or remove capacitance in order to re-tune the coil underdiffering load conditions, it has been determined that it would beuseful for the system to be re-tunable for a 10% change in tuningcapacitance. In this particular application, the coarse tuningincrements in capacitance were selected to be approximately 80 pF, andin the fine tuning mode, 10 pF. The RF signal source and amplifier (23,24) of RF sub-system 22 used to exercise the auto-tune subsystem are thesame as those used to excite the RF coil for substance detectionpurposes. Details of the auto-tune subsystem follow.

The apparatus of the invention for automatic fine tuning of the NQRdetection coil/head under adverse conditions is shown in detail in FIG.2. Within sequence controller 21 is software or control programming 91for auto-tune subsystem 36. The auto-tune subsystem is preferablyincorporated within RF shield 37, as are RF coil 34 and matching network35. Input/output line 92 connects the tuned RF coil to the amplified RFexcitation signal and connects the coil as the receiver of the NQRsignals to 1/4 wave line 38 (FIG. 1).

The system consists of a series of fixed value capacitors 93 switched byan equal number of vacuum relays 94. The amount of capacitance switchedinto the tuning circuit is determined by measuring the amount of powerbeing transferred from RF amplifier 24 to RF detector coil 34 (or, moreprecisely, the amount of "forward" to "reflected" power.) The means tomeasure this power transfer efficiency consist of a variety of common RFtechniques. For one application, a directional watt meter is used tomeasure the amount of "forward" to "reflected" power. Based on the powertransfer efficiency, capacitors are switched in or out of the circuit tomaximize power transfer efficiency from the RF amplifier to the RF coil.The system is thus re-tuned to provide the most efficient and mostsensitive RF coil. Once the state of tune of the RF coil has beendetermined by the values of the forward and reflected power, the coil isre-tuned by switching in capacitance according to the algorithmdescribed below.

Tuning of the RF coil consists of two stages: coarse tuning and finetuning. A flow diagram for the sequence is shown in FIG. 3. The value of"C" in FIG. 2 has been chosen to be 10 pf, so each capacitor is amultiple of "C." Other values could be assigned as desired.

Coarse Tuning

Both the forward and reflected power are measured. If reflected power isgreater than a predefined percentage of forward power, then the systemself-adjusts to coarse tuning by making the large jumps mentioned above(by increasing capacitance until reflected power drops below the maximumvalue of reflected power that the fine tuning mode can handle). Whenthat condition is reached, then the system goes into fine tuning. Theupper limit of the size of the capacitance jump is determined by thecapacitance range of the fine tuning subsystem. When the reflected powerdrops below a preset upper size limit, then the system will begin finetuning. This is the "start fine tuning mode" point.

Fine Tuning

After taking a step (either increasing or decreasing capacitance) thereflected power is measured again. If reflected power has increased andthe direction (i.e., increase or decrease of capacitance) has beenreversed from the previous step, then the system goes back one step tothe "start fine tuning mode" point. Fine tuning begins again, only thistime in the opposite direction (that is, adding capacitance instead ofsubtracting it). If, however, the reflected power did not increase, thenanother step is taken in the same direction (adding or subtractingcapacitance). This process continues until another reversal of thedirection is encountered. At this stage the system goes back one stepand the fine tuning is complete. The reflected power is now at aminimum. The forward power is measured and compared to a pre-definedvalue, to ensure correct functioning of the RF transmitter.

Auto-tune subsystem 36 performs two major functions. One is to re-tunethe RF coil to provide optimum performance under a range of coil loadingconditions. Secondly, it determines the state of the tune by comparingit to a pre-defined "zero" setting. The system consists of a radiofrequency power source, a directional watt-meter and switched capacitorsto vary tuning reactance. Control unit 21 operates the RF power source,measures the reflected power and then varies the tuning reactance untila minimum in reflected power is reached. The system's ability to tunethe sample coil directly results in increased overall efficiency.Antenna tuning systems commonly used in radio electronics areunnecessarily complex for coil fine tuning in NQR and NMR applications.They also have certain inefficiencies for NQR and NMR applications: theycannot tune the coil directly, and they experience higher feed linelosses, which can contribute to noise. Furthermore, antenna tuningsystems tend to be too general in terms of what is being matched (forexample, tuning range).

To further describe the detection system of FIG. 1, RF probe 35 is amatching network and Balun which provides tuning and matching of thecoil, and also protects preamplifiers 25 from the high voltages in thecoil during RF excitation. RF probe 35 matches RF coil 34 to a 50Ωunbalanced input. This makes the coil look like a 50Ωtransmitter/receiver and is conventional matching technology. Thefunction of 1/4 wave line 38 is to isolate the receiver from thetransmitter. Transmitter isolation diodes 39 and 40 have a relatedfunction. The auto-tune subsystem determines the state of the tune of RFcoil 34 in the detector head by matching the RF coil to its load in thedetection volume. It measures the amount of power transferred directlyto the RF coil (the "forward" power), and the amount of power reflectedback due to losses in the circuit and mis-tuning (the "reflected"power). Once the tuning state is determined by comparing the values ofthe forward and reflected powers, the coil is re-tuned by switchingcapacitance according to a predetermined sequencing as has beendiscussed in above.

When coil 34 is loaded with a package of unknown contents, it becomesde-tuned. In one application of this invention, to re-tune the coil,eight vacuum relays switch the capacitors arranged in pF values ofpowers of two, that is, 10, 20, 40, 80. This particular arrangement iscapable of producing 256 values of capacitance for re-tuning tuning thesystem, with a maximum total of 3000 pF. Rather than overloading thesystem with one relay for each value of capacitance, this powerarrangement minimizes the number of relays needed to produce a givenvalue of capacitance (eg. 10+20=30; 20+80=100, etc.), and affords veryfast operational speed. It should be noted that the same algorithm canbe used with a continuously-variable capacitance system. A stepper motorcould be employed and the actual tuning sequence would be very similarto that described for discrete, direct capacitor tuning. The direct coiltuning capacitance arrangement described above is preferred for thisinvention.

Using capacitors switched by vacuum relays requires a "settling time" ofabout 6 ms or less to allow the relays to operate and for the reflectedpower to achieve a steady-state value. The benefit in overall systemruggedness, efficiency, reliability, and small size due to the fixedswitch capacitor scheme overcome any possible advantage in precisiontuning which might have been achieved using the more conventionalvariable capacitors. However, because the system uses switching commandscontrolled by a computer operated sequence controlling device, it canget exact information on the amount of system de-tuning.

This tuning apparatus of the detection system offers improvedsensitivity for NQR systems by optimum automatic fine tuning of thesample coil (RF coil). Previous developments in coil fine tuningrequired manual tuning of the system, which is acceptable for thelaboratory but undesirable for field use. This system offers theadvantage of automatic tuning based on fixed capacitors switched byvacuum relays (designed for high RF switching) rather than bulkier andslower variable capacitors. The proposed system measures changes in coilloading, a feature not available on other detection systems. The systemis faster and easier to use than a manually tuned sample coil, andprovides information about the state-of-tune of the RF coil which cangive an indication of the contents of the coil (the sample). The systemalso gives the control unit an indication of the performance of the RFamplifier.

Once the auto-tuning procedure has been completed, the scanningprocedure begins. The scanning procedure is standard for detecting NQRsignals in real-world detection applications. In one application of thisinvention, the procedure consists of a combination of RF pulses,commonly known as PAPS (phase-alternated pulse sequence) and NPAPS(non-phase-alternated pulse sequence) versions of the SSFP (steady statefree precession) pulse sequence. These sequences are described in U.S.Pat. No. 5,365,171, which is incorporated herein by reference to theextent necessary for full explanation. However, there are othersequences of RF pulses which are commonly used in NQR procedures whichare also applicable for use in this invention. These are known andreadily useable by those of ordinary skill in this technical field.

When the test is commenced, the coil is tuned as described and scanningof the package is accomplished and at least one of the lights isilluminated. White light 101 flashes while tuning and testing are beingcompleted. Illumination of green light 102 indicates that no contrabandbeing tested for is present. Illumination of red light 103 indicatesthat the target substance has been found in such a quantity as to besignificant. If yellow light 104 is illuminated, it means there may besomething present which should be looked at or further tested. It couldmean there is a significant amount of metal present. Both yellow andgreen lights illuminated means there was no clear NQR signal and therewas metal or other conductive material present. Both red and yellowlights illuminated indicates that the target substance may be present,but it is at least partially obscured by metal. Those are indeterminateresults. Not shown is an ON/OFF button on a non-visible side of unit 81.

One challenge which must be overcome in proceeding from the laboratoryto a practical NQR detection system for scanning airline baggage is thatof acoustic ringing. A standing wave can be set up in a conductor placedin a pulsed RF field. This acoustical wave is picked up by the RF coil.The signal produced is often close to the same magnitude andsufficiently close in characteristics to an NQR signal to possibly causea false alarm. The acoustical signal is often coherent with the excitingRF pulse, and hence can potentially be mistaken for an NQR signal, whichis also coherent with the exciting RF pulse. Moreover, common methodsfor reducing spurious ringing effects in laboratory NQR systems, such assignal averaging and/or reversing the RF phase, will often notsufficiently reduce the problem. Certain types of commonly-occurringmaterials, such as spring steel, are particularly prone to acousticringing.

In the preferred embodiment of this invention, a simple but effectivemethod of reducing the effects of acoustic ringing in NQR detectionapplications is employed. The primary differing characteristic of an NQRsignal compared with an acoustic ringing signal is that NQR signalsoccur only at pre-defined frequencies. Acoustic ringing signals, on theother hand, can be generated by any frequency of an RF excitation pulse.Thus, by operating the NQR scanning system at a frequency outside therange of the NQR sample frequency, using a standard or modified RF pulsesequence, no signal will be generated by or be detected from any targetmaterial. Under these conditions, if a signal is seen, it is fromacoustic ringing. Implementation of this method is straightforward inthe tuning system of the invention. The "ring detect" sequence can beimplemented before or after the main sample detect sequence and is partof the programming and RF signal generation. This frequency excursioncan easily be provided by the auto-tune aspect of this invention.

In the scanner system of FIG. 1, when employing analog detectors, signalcapture and data processing subsystem 41 comprises two analog to digital(A/D) converters 42 and 43 and digital signal processor 44. The receivedsignals from phase sensitive detectors 27 and 28 are fed to A/Dconverters 42 and 43 respectively. All signals produced by the samplescan and ring detect sequences are fed into the A/D converters and areprocessed by the digital signal processor. Through the sample scansequence, signals are either added or subtracted, according to thealgorithm outlined in U.S. Pat. No. 5,365,171. The addition/subtractionalgorithm reduces the effects of RF coil ring-down and magnetoacousticringing.

In a practical configuration of this portion of the invention, signalcapture and most of the signal processing is carried out on a plug-in PCA/D converter card. The card has two channels, 14-bit resolution, and a2 MHz sampling rate. Subsystem 41 also performs on-board digital signalprocessing functions, such as addition or subtraction of consecutivedata sets as required. Once processing the output signal is completed,it is digitally filtered and compared to a predefined threshold level.Alternatively, once the signal is apodized and Fourier-transformed, itoccurs as a quadrature "spike" at or close to 0 Hz in the frequencyspectrum, and is then filtered and compared to the "known" signal of thematerial to be detected.

In the frequency domain, the signal capture and data processingsubsystem compares other signal factors to the expected signal factors.For example, it may compare the signal shape (Lorentzian or Gaussian) tothe line-width at half height. A combination of the above signal factorsmay be used to determine the presence or absence of the targetsubstance. The output of the digital signal processor is then sent todisplay device 46.

The NQR detected signal is compared with a predetermined threshold levelstored in memory in digital signal processor 44. If the detected signalis equal to or greater than the predetermined threshold, red light 103flashes on the operator's panel on display device 46, indicating thepresence of the target substance. If the signal is less than thepredetermined threshold, green light 102 flashes, indicating the absenceof the target substance. If the auto-tune algorithm detects that anexcessive amount of re-tuning of the coil is necessary, compared to anaverage investigation or predefined threshold, or an acoustic ringingsignal is detected, the condition is flagged and yellow warning light104 illuminates. The yellow warning light indicates that: (1) there isan abnormally high amount of metal in the coil, (2) a high quantity ofhigh dielectric material is detected, or (3) a spurious acoustic signalhas been detected. Further alternative testing or visual inspection canbe used to resolve inconclusive results of the NQR test.

In addition to the illumination indications mentioned above, the displaydevice can optionally provide graphical display 95 of the signal showingboth the in-phase and quadrature signals, as well as other signal andsystem characteristics. Also optionally, printed output 96, includingthe time, date, signal amplitude and frequency, as well as coil tuningparameters, and other information such as acoustic signal responses fromspeaker 97, can be provided.

The factors which have degraded the effectiveness of previous NQR signaldetectors are reduced or eliminated by this system. If conductive orhigh dielectric materials are present in the sample, the auto-tunesub-system will be employed in an attempt to neutralize the effect ofthe foreign material. Then visual inspection can be accomplished ifthere is reason to do so. The auto-tune capability can quickly accountfor changes in temperature which affects tuning capacitance, as well asmovement or distortion of the coil which might occur when samples areput into the cavity.

The tuning system of the invention has been described as a specificcomponent of an NQR scanner system. Because NMR detector systems alsorequire variable RF frequencies to be applied and detected, thisinvention has direct application to those detector systems as well. Theautomatic tuning function of this invention is equally applicable to NQRand NMR detection systems.

This tuning apparatus offers improved sensitivity for NMR and NQRsystems by optimum automatic fine tuning of the sample coil. Both thealgorithm used and the apparatus designed to implement the algorithmrepresent a novel departure from current algorithms and apparatuses inthe field of NQR and NMR. Furthermore, no known prior art apparatus isas efficient as the proposed system, nor is any capable of displaying orpresenting information on the current status of the tune. The proposedsystem allows for a wide range of impedance matching and the maintenanceof a resonant frequency.

An embodiment of the invention has been described above. It is likelythat modifications and improvements will occur to those skilled in thistechnical field which are within the scope of the appended claims.

I claim:
 1. An automatic tuning apparatus to establish maximum power transfer efficiency at at least one selected predetermined frequency in nuclear quadrupole and nuclear magnetic resonant (NQR/NMR) detection systems when a specimen is inserted in the detection element of an NQR/NMR detection system in order to enable such detection systems to detect nuclear quadrupole or nuclear magnetic resonant frequencies of predetermined substances if present in the specimen, the predetermined substances having predetermined characteristic nuclear resonant frequencies, wherein nuclear resonant frequency is the frequency of nuclear precession due either to quadrupolar interation with molecular electric field gradients (NQR) or to an applied static magnetic field (NMR), said apparatus comprising:a sequence controller having means for providing timing and programming pulses to said apparatus; a variable frequency RF source to provide pulsed RF excitation at a predetermined frequency generally corresponding to the nuclear resonant frequency of a predetermined substance; a distributed RF coil sheet shaped and configured to define a cavity of predetermined volume therewithin and to receive the specimen within the cavity defined by said coil, the RF signals from said RF source being transmitted within said cavity and being applied to the specimen within said RF coil cavity and generating a field within said cavity, said RF coil also functioning as the pickup coil for the NQR/NMR signals from the specimen and providing an output signal; means for measuring the power transfer efficiency of said RF coil at the predetermined nuclear resonant frequency; and adaptive tuning apparatus connected in said apparatus, said adaptive tuning apparatus being responsive to the measured field to tune said RF coil for maximum power transfer efficiency; said sequence controller controlling said adaptive tuning apparatus pursuant to power transfer efficiency measurements from said measuring means to establish and maintain maximum power transfer efficiency of said RF coil at the predetermined nuclear resonant frequency.
 2. The apparatus recited in claim 1, wherein said means for measuring power transfer efficiency of said RF coil comprises means for measuring the amount of power transferred directly to said RF coil from said RF source (forward power), and measuring the amount of power reflected back into said coil (reflected power).
 3. A method for automatically tuning apparatus to establish maximum power transfer efficiency at a predetermined frequency in a coil when a specimen is inserted into a cavity formed by said coil to enable said coil to detect nuclear quadrupole or nuclear magnetic resonant (NQR/NMR) frequencies of predetermined substances if present in the specimen, the predetermined substances having predetermined characteristic nuclear resonant frequencies, wherein nuclear resonant frequency is the frequency of nuclear precession due either to quadrupolar interaction with molecular electric field gradients (NQR) or to an applied static magnetic field (NMR), said method comprising the steps of:shaping and configuring an RF coil sheet to define the cavity having a predetermined volume therewithin, the cavity being adapted to receive specimens to be tested for the presence of the predetermined substances; inserting the specimen within the cavity formed by the RF coil; providing RF pulses at about one said predetermined characteristic nuclear resonant frequency to the RF coil to establish a field within the cavity in said coil which encompasses the specimen to thereby apply RF signals to the specimen within the RF coil cavity; measuring the power transfer efficiency of the coil at the predetermined nuclear resonant frequency; and tuning to said RF coil by means of adaptive tuning apparatus coupled thereto, said adaptive tuning apparatus being responsive to the measured field in the coil to tune the RF coil in accordance with a feedback algorithm to establish maximum power transfer efficiency of the RF coil at the predetermined nuclear resonant frequency.
 4. The method recited in claim 3, wherein the step of measuring the power transfer efficiency comprises the steps of:measuring the amount of power transferred directly to the RF coil (forward power); measuring the amount of power reflected back to the RF coil (reflected power); and adjusting the adaptive tuning apparatus in accordance with a feedback algorithm to establish maximum power transfer efficiency of the RF coil at the predetermined nuclear resonant frequency.
 5. The method recited in claim 3, wherein controller apparatus provides the feedback algorithm controlling the adaptive tuning apparatus. 